Confocal laser microscopes perform imaging by scanning a focused laser beam over the surface of the target to be viewed. FIG. 1 is a block diagram of a confocal laser microscope. Laser 102 generates laser beam 118, which is transmitted to beam splitter 104, X-mirror 106, spatial filter 107, Y-mirror 108, and objective lens 110 to target 112. When the distance between objective lens 110 and target 112 is such that the microscope is in a focused condition, laser beam 118 is reflected from target 112, back through objective lens 110, Y-mirror 108, spatial filter 107, X-mirror 106, and beam splitter 104 to detector 114. When the microscope is not in a focused condition, only a small portion of laser beam 118 is reflected to detector 114. Detector 114 generates an imaging signal 116 which is representative of the intensity of laser beam 118 reflected to detector 114. Imaging signal 116 is transmitted to microprocessor 120. Microprocessor 120 processes imaging signal 116 to create a video image signal 121 which is transmitted to video display terminal 122. Video display terminal 122 displays the image of target 112. Microprocessor 120 also controls other functions within the microscope.
FIG. 2 is a top view of target 112 illustrating the imaging of an area 202 of target 112. To obtain an image of target area 202, X-mirror 106 and Y-mirror 108 are deflected to scan the laser beam 118 along a path 204 which follows a series of rows within target area 202. In this manner, detector 114 receives imaging information for target area 202. Target area 202 is parallel to the X-Y plane.
FIG. 3 is a side view of target 112, illustrating laser beam 118 at three positions 301-303 along path 204. Confocal microscopes typically have a narrow focal plane 307 along the Z-axis. Surfaces of target 112 positioned outside of focal plane 307 fail to reflect a significant portion laser beam 118 from target 112 to detector 114. Thus, a small imaging signal 116 is generated when laser beam 118 is at position 302 because surface 305 is outside of focal plane 307. Consequently, the resulting image of surface 305 appears dark, rather than blurry. This results in an imaging signal 116 which only represents surface 305 at a single plane (i.e., a single frame). Certain targets, such as semiconductor wafers, can have uneven surfaces such as surface 305. To accurately represent surface 305, imaging signal 116 is therefore generated at many focal planes to obtain the information necessary to image the surface 305 of target 112.
It is therefore desirable to have a confocal microscope capable of generating an imaging signal 116 which represents a plurality of focal plane images (i.e., frames) of a target having a varying surface terrain. It is also desirable to have a method and apparatus for processing these frames of information to create an image representative of the surface of the target 112.
In addition, when imaging signal 116 is being transmitted to microprocessor 120, the bandwidth of the input/output (I/O) bus of microprocessor 120 is almost entirely consumed by the transfer of image data and therefore cannot be used to receive or transmit other information to control the microscope. Also, because imaging signal 116 contains a large amount of information, it takes a significant amount of time for microprocessor 120 to process imaging signal 116. It is therefore desirable to avoid transmitting imaging signal 116 to microprocessor 120 when generating a video image on video display terminal 122.